Printed circuit boards are manufactured from a sheaf of sheets of fibreglass fabric, fibreglass paper or the like, impregnated with a resin, such as epoxy, phenolic or polyester resin, etc. One or both outer sides of this sheaf incorporate one or two sheets of copper, which will serve to form the tracks of the printed circuit.
Once formed, the sheaf is subjected to heat and pressure in order to cure the resin and obtain a compound laminate, from which the boards for printed circuits are cut to the desired size.
Various compound laminates exist, and the materials of the base sheets and the resins used for impregnating the base sheets can be varied, depending on the application they are to be used for. The present invention relates more particularly to compound laminates made up of sheets of fibreglass fabric and sheets of fibreglass paper impregnated with epoxy resins.
Two of these compound materials widely used are described for example in US standard NEMA LI-1, and are known in this field of the art under the designations "CEM-3" and "FR-4".
CEM-3 is a compound laminate which includes two outer sheets of continuous-strand fibreglass fabric and a core of non-woven fibreglass (made up of sheets of fibreglass paper), all the sheets being impregnated with epoxy resin; this is the closest prior art to the present invention, and it serves as a basis for the preamble of claim 1.
FR-4 is a compound laminate made up only of sheets of fibreglass fabric impregnated with an epoxy resin.
FR-4 is a material with good thermal stability (the dilation due to an increase in temperature is low), but fibreglass fabric has a high cost and, moreover, the laminate is difficult to die-cut because of its hardness. As a consequence, this material is expensive and is normally reserved for the field of professional electronics.
CEM-3 is lower in cost and is easier to die-cut; it nevertheless presents some characteristics which may be critical for some applications. In the first place, its dimensional stability under thermal stresses is lower than that of FR-4, and in some cases thermal dilation of the board can lead to breakage of the soldered points. This phenomenon arises particularly when using surface-mounting ceramic components and integrated circuits, known as "SMD" (surface mounting device), especially where such devices are of considerable length.
In consequence, where SMD ceramic components of large size must be incorporated into the circuit, there may be a need to use FR-4 boards, thereby making the printed circuits more expensive. It would be desirable to avoid such increased cost, especially in consumer and semi-professional electronics, fields in which increasing numbers of SMDs are being used.
Another disadvantage of CEM-3 is that it is more fragile in the process of grooving the working panels in individual circuits, by means of the "scoring" technique, since the formed groove can totally eliminate the layer of fibreglass, so that the impregnated paper left is not strong enough to prevent undesired breakage of the panel.